With a long-term objective to understand the developmental stages of the B cell response to antigen, this research will focus on germinal center and memory B cells, which relate to the cardinal features of immunity, affinity maturation of the immune response and immunological memory. The specific aims are to determine: 1) the role of heparan sulfate/heparin in serving as a ligand for CD19 in the germinal center; 2) whether developmental down-regulation of SHP-1 in the germinal center B cell is required, and whether this permits signaling through a gamma-c cytokine receptor; 3) whether arrested terminal differentiation by BCL-6 is the basis for B cell memory; 4) whether signaling through a gamma-c, cytokine receptor is necessary to maintain memory B cells; and 5) whether B cells with dysregulated c-myc present as centroblast-like Burkitt lymphomas because of unique expression patterns of BCL-6 and SHP-l at this stage of development. For Aims 2-5, mice will be created in which transgenes can be inducibly expressed in a lineage-specific manner. A bacterially-derived transactivator transgene, rtTA, will be expressed only in B cells and will drive transcription of "target" transgenes in the presence of doxycycline. The target transgenes are SHP-1, dominant negative (DN) gamma-c, and wild-type and DN BCL6, all linked via an IRES to EGFP to permit visualization of transgene expression. With doubly transgenic, Fl mice, giving doxycycline will 1) up-regulate SHP-1 and DN gamma-c in germinal center B cells, allowing analysis of the role of low SHP-1 and cytokine receptor signaling in these cells; and 2) up-regulate DN gamma-c, and wild-type and DN BCL-6 in memory B cells, allowing analysis of the role of BCL-6 and cytokine receptor signaling in these cells. The understanding gained from these studies of normal B cell development will be applied to a murine model of Burkitt lymphoma to discover why translocation of c-myc at an early stage of B cell development leads to a B cell tumor at a much later stage.